1. Field of the Invention
The invention relates to an organic EL (electroluminescent) display device, particularly to an organic EL display device in which a plurality of pixels is disposed on a device substrate to form a pixel region and each of the pixels has an organic EL element and a driving transistor for driving the organic EL element.
2. Description of the Related Art
In recent years, an organic EL display device using organic EL elements has been receiving attention as a new display device to replace a CRT or an LCD. For example, research and development of an EL display device having thin film transistors (hereafter, referred to as TFTs) as switching elements for driving the organic EL elements are being pursued.
FIG. 7 is a partial plan view of such an organic EL display device, and FIG. 8 is a cross-sectional view thereof. A pixel region 200 is disposed on a device glass substrate 100, and a horizontal drive circuit 250 and a vertical drive circuit 260 serving as a drive circuit are disposed on a periphery of the pixel region 200. The vertical drive circuit 260 supplies a gate signal Gn as a pixel selecting signal to each of the pixels in the pixel region 200. The horizontal drive circuit 250 supplies a display signal Dm to each of the pixels in the pixel region 200 based on a horizontal scanning signal. Each of the vertical and horizontal drive circuits is configured of shift resistors.
The plurality of pixels is disposed in a matrix in the pixel region 200. FIG. 7 shows a pixel GS only. A structure of this pixel GS will be explained as follows. A gate signal line 201 supplying the gate signal Gn and a drain signal line 202 supplying the display signal Dm intersect each other. An organic EL element 203, a driving TFT 204 for driving the organic EL element 203, and a pixel selecting TFT 205 for selecting the pixel GS are disposed on a periphery of an intersection of these signal lines.
A drain 204d of the driving TFT 204 is supplied with positive power supply voltage PVdd. A source 204s of the driving TFT 204 is connected with an anode of the organic EL element 203. A gate of the pixel selecting TFT 205 is connected with the gate signal line 201, and supplied with the gate signal Gn from the gate signal line 201. A drain of the pixel selecting TFT 205 is connected with the drain signal line 202, and supplied with the display signal Dm from the drain signal line 202. A source of the pixel selecting TFT 205 is connected with a gate of the driving TFT 204.
The organic EL element 203 includes an anode, a cathode, and an emissive layer formed between the anode and the cathode. The cathode is supplied with negative power supply voltage CV.
Furthermore, the gate of the driving TFT 204 is connected with a storage capacitor 206. That is, one electrode of the storage capacitor 206 is connected with the gate of the driving TFT 204, and another electrode thereof is connected with a storage capacitor electrode 207. The storage capacitor 206 stores the display signal Dm applied to the gate of the driving TFT 204 through the pixel selecting TFT 205 for a field period by storing electric charge corresponding to the display signal Dm.
Operation of the EL display device having the above-described structure will be described. Here, the driving TFT 204 is of P-channel type, and the pixel selecting TFT 205 is of N-channel type.
When the gate signal Gn is high level for a predetermined horizontal period, the pixel selecting TFT 205 turns on. Then, the display signal Dm is applied from the drain signal line 202 to the gate of the driving TFT 204 through the pixel selecting TFT 205. According to the display signal Dm supplied to the gate, conductance between the source and the drain of the driving TFT 204 changes. A drive current corresponding to the changed conductance is supplied to the organic EL element 203 through the driving TFT 204, thereby exiting the organic EL element 203.
The organic EL element 203 degrades its characteristics by absorbing moisture. Therefore, as shown in FIG. 8, the above-described device glass substrate 100 and the sealing glass substrate 300 are attached to each other by using sealing resin 301 made of, for example, an epoxy resin. Furthermore, a concave portion 302 is formed on a surface of the sealing glass substrate 300, which is on the side facing the device glass substrate 100, and a desiccant layer 303 is attached on a bottom of the concave portion 302. This technology is disclosed in the Japanese Patent Application Publication No. 2002-175029.
As shown in FIGS. 7 and 8, an organic interlayer insulating film 208 is formed on and covers the driving TFT 204. The source 204s of the driving TFT 204 is connected with the anode of the organic EL element 203 through a contact hole provided in this organic interlayer insulating film 208. The organic interlayer insulating film 208 has appropriate characteristics as an interlayer insulating film, since it can be formed thick with its low stress and permittivity, and also costs low. On the other hand, however, the organic interlayer insulating film 208 has an adverse characteristic, i.e., high moisture transmittance.
Therefore, moisture infiltrating through the sealing resin 301 from outside of the organic EL display device partially reaches the pixel region 200 through this organic interlayer insulating film 208, thereby degrading the characteristics of the organic EL element 203.